1. Field of the Invention
The present invention generally relates to a photoelectric smoke detecting apparatus (also known as the smoke detector) for generating analog data concerning smoke density indicating occurrence of fire or the like event with the aid of a microcomputer or microprocessor. More particularly, the present invention is concerned with a photoelectric smoke detecting apparatus which is imparted with a self- or auto-compensation capability of automatically or spontaneously compensating for a time-dependent change or aged deterioration of a detection characteristic (light reception sensitivity) of a light receiving element incorporated in a smoke sensor of the smoke detecting apparatus due to contamination thereof.
2. Description of Related Art
Heretofore, a well known type of photoelectric smoke detecting apparatus is arranged such that a light emitting element is disposed within a well-ventilated chamber of a smoke sensor and is electrically driven periodically at a predetermined time interval for enabling a microcomputer or microprocessor to fetch the detection signal from the output of the smoke sensor thereby processing the same in order to decide whether a fire event is taking place in a place where the smoke sensor is installed or to detect the density of smoke prevailing in that place.
More specifically, the detection signal outputted from the light receiving element of the smoke sensor disposed for receiving light rays scattered by smoke particles is amplified by an amplifier circuit provided in association with the smoke sensor. The amplified signal is supplied to a microcomputer or microprocessor after analog-to-digital conversion (A/D conversion), whereon the digital data as fetched by the microcomputer is converted to corresponding smoke density data, which is then sent out in the form of an analog data signal to receiver equipment installed at a center station.
In the photoelectric smoke detecting apparatus of this type, contamination of an inner wall of a casing, a light emitting element and/or the light receiving element which constitute the smoke sensor will bring about a variation or change in the sensitivity characteristic of the smoke sensor and hence a change in the detection signal level which of course depends on the color of a contaminant.
Thus, when the contamination of the smoke sensor is detected, there arises the necessity of cleaning the sensor in order to restore the original state thereof to thereby prevent erroneous or false detection of the fire state. When such cleaning is difficult or practically impossible for some reason, it will then be required to take other appropriate measures such as exchange of the smoke sensor itself.
For having better understanding of the concept underlying the present invention, description will first be directed to the conventional photoelectric smoke detecting apparatus known heretofore by reference to FIGS. 6 and 7 of the accompanying drawings. FIG. 6 is a functional block diagram showing schematically a structure of a conventional photoelectric smoke detecting apparatus, whereas FIG. 7 is a circuit diagram of the same.
Referring to FIG. 6, the conventional photoelectric smoke detecting apparatus includes a smoke sensor 10 which is composed of a light emitting element 11 and a light receiving element 12. A shielding plate 13 is interposed between the light emitting element 11 and the light receiving element 12. It is noted that the light emitting element 11, the light receiving element 12 and the shielding plate 13 are disposed within a chamber enclosed by a labyrinth inner wall 14 which is employed for implementing the smoke sensor in an antireflection structure. By virtue of this structure, the light receiving element 12 can receive only the scattered light rays L2 of the light rays L1 emitted by the light emitting element 11, whereby the detection value D indicating the smoke density within the chamber enclosed by the labyrinth inner wall 14 can be acquired in the form of a detection signal outputted from the smoke sensor 10.
A control unit 20 which may be constituted by a microcomputer or microprocessor is designed or programmed to process the detection signal D outputted from the smoke sensor 10 to thereby output an analog data signal E indicative of the smoke density prevailing within the smoke sensor 10. At this juncture, it should be mentioned that a plurality of photoelectric smoke detecting apparatuses each composed of the smoke sensor 10 and the control unit 20 may be disposed at various locations within a building or the like where the smoke detection is required.
The output data signals (analog data signals E) of the individual photoelectric smoke detecting apparatuses installed at various places are supplied to receiver equipment 30 installed at a center station through signal transmission via signal lines (not shown).
As can be seen in FIG. 6, the control unit 20 includes a driving circuit 21 for generating a driving pulse signal P for driving the light emitting element 11, and A/D (analog to digital) converter 22 for converting the detection value D into digital data Dd and a smoke density arithmetic module 23 for determining arithmetically a smoke density value VKe on the basis of the digital data Dd by referencing a characteristic function table 23T incorporated in the smoke density arithmetic module 23. The control unit 20 is provided with a sender or transmission circuit 24 for sending or transmitting the smoke density value VKe in the form of an analog data signal E to the receiver equipment 30 of the center station.
In the characteristic function table 23T, there are stored characteristic functions each approximated by a positive linear function (represented by a straight line), as described later on.
Next, description will be made by reference to FIG. 7 in which reference characters 10 to 13, 20, L1 and L2 denote same items as those described above by reference to FIG. 6.
Referring To FIG. 7, the microcomputer 40 constituting a major part of the control unit 20 includes a CPU (Central Processing Unit) which serves for the functions of the A/D converter 22 and the smoke density arithmetic module 23 shown in FIG. 6 and other peripheral components. A light emitting circuit 41 corresponds to the driving circuit 21 shown in FIG. 6 and serves for electric power supply to the light emitting element 11 as well as pulse-like light emission control thereof. A light receiving circuit 42 is electrically connected to the light receiving element 12, and an amplifier circuit 43 is connected to the output of the light receiving circuit 42 for amplifying the detection signal, the amplified detection signal being then inputted to the microcomputer 40.
An oscillator circuit 44 is provided for supplying a clock pulse signal CK to the microcomputer 40. Further provided is an EEPROM (Electrically Erasable Programmable Read-Only Memory) 45 which is connected to the microcomputer 40 for storing preset data such as addresses and others.
An alarm lamp 46 is provided as an alarming means for generating an alarm upon occurrence of abnormality such as a fire. The alarm lamp 46 is driven or electrically energized by a lighting circuit 47 under the control of the microcomputer 40.
A receiving circuit 48 serves for receiving signals such as external signals sent from the receiver equipment 30 (see FIG. 6), which signal are then inputted to the microcomputer 40. On the other hand, the output signals of the microcomputer 40 are sent to external apparatus via a transmitting circuit 49. Incidentally, the receiving circuit 48 and the transmitting circuit 49 functionally correspond to the transmission circuit 24 shown in FIG. 6.
A constant-voltage circuit 50 is provided for supplying electric power to the microcomputer 40 and others incorporated in the control unit 20 and other discrete circuits 41 to 49.
A diode bridge circuit 51 serves for nullifying the poralities of terminals when the control unit 20 and the receiver equipment 30 of the center station (see FIG. 6) are interconnected by a signal line (not shown).
FIG. 8 is a signal waveform diagram for illustrating detection levels or pulses outputted from the light receiving element 12 in correspondence to the driving pulses P, respectively, in the state where the smoke density is zero when the driving pulses P are applied to the light emitting element 11.
As can be seen in FIG. 8, a train of driving pulses P includes first pulses PI for fire detection and a second pulse P2 for fault detection, wherein the second pulse P2 is at a higher level than the first pulse P1.
At this juncture, it should be mentioned that the second pulse P2 serves for the function for increasing or intensifying the light emission of the light emitting element 11 in addition to the function of the first pulse P1. As the alternative, the second pulse P2 may be generated by increasing intermittently the amplification factor of the amplifier circuit 43 connected to the output of the light receiving circuit 42.
The output period or cycle xcfx84 of the first pulses P1 and the second pulses P2 is set at an equi-interval (e.g. two seconds), wherein the second pulse P2 for fault detection is generated once for four pulses (e.g. at the interval of eight seconds).
With the conventional photoelectric smoke detecting apparatus of the structure described above by reference to FIGS. 6 and 7, the smoke sensor 10 is driven in response to the driving pulse train P illustrated in FIG. 8, whereby emission of light rays L1 and reception of the scattered light rays L2 are carried out repetitively, as a result of which the detection value D is outputted from the light receiving element 12.
On the other hand, the control unit 20 fetches the detection value D through the medium of the light receiving circuit 42, the amplifier circuit 43 and the A/D converter 22 to thereby generate the analog data E indicative of the smoke density in accordance with the characteristic function table 23T, the analog data signal E as generated being then sent to the receiver equipment 30 via the transmitting circuit 49 as shown in FIG. 7 (corresponding to the transmission circuit shown in FIG. 6).
Since the second pulse P2 is contained in the driving pulse train P, the light emitting element 11 emits the light rays L1 at a higher output level once for eight seconds. In response to the emitted light rays L1 of the high intensity, the light receiving element 12 outputs the detection value D which can be used for detecting the noise level internally of the smoke sensor 10.
At this juncture, it should be added that the characteristic function stored in the characteristic function table 23T remains unchanged in the initial state without being corrected even when the characteristic function of the smoke sensor 10 has changed.
According to the International Standards FDK38U as well as the Japanese Standards FDK038-X, it is recommended that the fire detection or fault detection be performed at the output period xcfx84 of about two seconds and that the fault detection be performed once for four cycles (i.e., periodically at an interval of about eight seconds).
As is apparent from the foregoing description, in the photoelectric smoke detecting apparatus known heretofore, no compensating measures are taken or adopted against the change of the detection level. Consequently, when the characteristic function of the smoke sensor has changed, the analog data E indicating accurately the smoke density can no more be made available, giving rise to a problem that the fire state can not be determined with reasonable accuracy and reliability in the center station equipped with the receiver equipment 30.
In the light of the state of the art described above, it is an object of the present invention to provide a photoelectric smoke detecting apparatus which is capable of making available the analog data signal indicating accurately the smoke density regardless of contamination of the smoke sensor by imparting to the photoelectric smoke detecting apparatus the function or capability for automatically or spontaneously compensating the time-dependent change of the detection value derived from the output of the light receiving element of the smoke sensor due to the contamination thereof.
In view of the above and other objects which will become apparent as the description proceeds, there is provided according to a general aspect of the present invention a photoelectric smoke detecting apparatus which includes a smoke sensor composed of a light emitting element and a light receiving element accommodated within a chamber enclosed by a labyrinth inner wall for outputting from the light receiving element a detection signal indicative of a detection value corresponding to a smoke density prevailing within the chamber enclosed by the labyrinth inner wall. The smoke detecting apparatus further includes a control unit for outputting analog data corresponding to the smoke density on the basis of the detection value. The control unit is comprised of a smoke arithmetic module having a characteristic function for converting the detection value to a smoke density value, a zero-density detection value storage device for storing a detection value at a time point when the smoke density is zero as a zero-density detection value, a change rate (i.e. ratio) arithmetic module designated for determining arithmetically a ratio of change (also referred to as the change ratio) of the zero-density detection value, and a compensation arithmetic module designed for compensating conversion characteristic for converting the detection value to the smoke density value by taking into account the above-mentioned ratio of change. Further, the compensation arithmetic module is designed as cause the smoke density arithmetic module to generate a smoke density value in such a manner that change of output characteristic of the detection value for the smoke density, which change bears dependency on ratio of the change, can be canceled out.
In a preferred mode for carrying out the present invention, the change ratio arithmetic module may be designed to arithmetically determine the change ratio as a value derived by dividing the zero-density detection value by an initial value thereof, wherein the compensation arithmetic module is so designed as to correctively increase the detection value as the change ratio of the zero-density detection value increases or alternatively decreases from a value xe2x80x9c1 (one)xe2x80x9d.
In another mode for carrying out the present invention, the change ratio arithmetic module should preferably be designed to arithmetically determine the change ratio in terms of an absolute value derived from division of a change quantity of the zero-density detection value from the initial zero-density detection value by the initial value, wherein the compensation arithmetic module is so designed as to correctively increase the detection value in dependence on increasing of the change ratio of the zero-density detection value.
In yet another mode for carrying out the present invention, the compensation arithmetic module should preferably be so designed as to correct the detection value in dependence on the change ratio and establish a detection value after compensation by adding or, alternatively, subtracting the change quantity of the zero-density detection value.
In still another mode for carrying out the present invention, the change ratio arithmetic module should preferable be designed to arithmetically determine the change ratio as a value derived by dividing the zero-density detection value by an initial value thereof, wherein the compensation arithmetic module is designed to correctively establish a slope of the currently valid characteristic function to be smaller than an initial slope thereof as the change ratio increases or alternatively decreases from a value xe2x80x9c1 (one)xe2x80x9d.
In a further mode for carrying out the present invention, the change ratio arithmetic module should preferably be designed to arithmetically determine the change ratio in terms of an absolute value derived from division of a change quantity of the zero-density detection value from the initial zero-density detection value by this initial value, wherein the compensation arithmetic module is designed to correctively establish a slope of the currently valid characteristic function to be smaller than an initial slope thereof in dependence on increasing of the change ratio.
In a yet further mode for carrying out the present invention, the compensation arithmetic module should preferably be designed to correct the slope of the characteristic function in dependence on the change ratio and establish a characteristic function after compensation by adding to or, alternatively, subtracting from the zero-density detection value the change quantity of the zero-density detection value.
In a still further preferred mode for carrying out the present invention, the control unit may include an analog-to-digital converter for converting the detection value to digital data, wherein the smoke density arithmetic module is designed to convert the digital data to the smoke density value.
In another preferred mode for carrying out the present invention, the compensation arithmetic module-may include a compensation range discriminating means for deciding whether the change ratio falls within a predetermined range for compensation and generating fault information when the change ratio departs from the predetermined range for compensation.
In yet another mode for carrying out the present invention, the compensation arithmetic module should preferably be designed that when a state in which the change ratio falls within the predetermined range for compensation has continued for a predetermined time duration, a value derived through average processing of the zero-density detection value over the predetermined time duration is employed as a final change ratio.
In still another preferred mode for carrying out the present invention, the compensation arithmetic module may include a compensating value setting module for fixedly placing therein a compensating value which corresponds to the change ratio.
In a further mode for carrying out the present invention, the compensation arithmetic module may preferably include a correcting value setting means for establishing a correcting value for correcting the compensating value in dependence on the zero-density detection value.
In a yet further mode for carrying out the present invention, the correcting value setting means may preferably include a correcting value storing means for storing the correcting value, wherein the correcting value can externally altered through input manipulation.
By virtue of the arrangements described above, there can be implemented a photoelectric smoke detecting apparatus which is capable of generating analog data which accurately indicates the smoke density regardless of contamination of the smoke sensor owing to the feature of self- or auto-compensation for aged deterioration or time-dependent change of the detection value outputted from the light receiving element of the smoke sensor due to contamination thereof
The above and other objects, features and attendant advantages of the present invention will more easily be understood by reading the following description of the preferred embodiments thereof taken, only by way of example, in conjunction with the accompanying drawings.